Future missions

The spectroscopic and photometric observations of hot Jupiters have provided a wealth of information on their physical characteristics and led to insights about their atmospheric structure. What about extrasolar planets similar to the Earth? Although detection of such rocky planets remains just beyond the limits of current techniques, a few short-period planets with masses only 5-15 times that of the Earth (sometimes called 'hot super-massive Earths') have been discovered

Wavelength (|m)

Fig. 2.5. Theoretical spectra of Earth. Upper panel: theoeretical model that matches the Woolf et al. (2002) Earthshine data. The dashed vertical lines show the nominal wavelength range of TPF-C. Lower panel: normalized models of Earth showing effects of clouds. The top curve is for uniform high cloud coverage showing weaker water vapour features. The bottom curve shows the case with no clouds, resulting in deep absorption features.

Fig. 2.5. Theoretical spectra of Earth. Upper panel: theoeretical model that matches the Woolf et al. (2002) Earthshine data. The dashed vertical lines show the nominal wavelength range of TPF-C. Lower panel: normalized models of Earth showing effects of clouds. The top curve is for uniform high cloud coverage showing weaker water vapour features. The bottom curve shows the case with no clouds, resulting in deep absorption features.

(Santos et al., 2004; Beaulieu et al., 2006), pushing the detection limit to ever smaller planets. We close this chapter with a brief discussion of how we might search for Earth-like planets around other stars and what future missions are being planned to tackle this fundamental question.

The goal of directly imaging an Earth-like planet is to search for biosignatures, which are spectral features that can be used as diagnostics to search for the presence of life as we know it. The Earth has several such biosignatures that are indicative of habitability or life. Figure 2.5 shows two of these species: O2 and its photolytic product O3, two of the most reliable biosignature gas indicators of life. O2 is highly reactive and therefore will remain in significant quantities in the atmosphere only if it is continually produced. There are no abiotic continuous sources of large quantities of O2 and only rare false positives that in most cases could likely be ruled out by other planetary characteristics. N2O is a second gas produced by life - albeit in small quantities - during microbial oxidation-reduction reactions. N2O has a very weak spectroscopic signature.

2.9 Summary

In addition to atmospheric biosignatures, the Earth has one very strong and very intriguing biosignature on its surface: vegetation. The reflection spectrum of photosynthetic vegetation has a dramatic sudden rise in albedo around 750 nm by almost an order of magnitude! (This effect is not included in the model plotted in Figure 2.5.) Vegetation has evolved this strong reflection feature, known as the 'red edge', as a cooling mechanism to prevent overheating which would cause chlorophyll to degrade. On Earth, this feature is likely reduced by a few per cent due to clouds. A surface biosignature might be distinguished from an atmospheric signature by observing time variations; i.e., as the continents, for example, rotate in and out of view, the spectral signal will change correspondingly. Other spectral features, although not biosignatures because they do not reveal direct information about life or habitability, can nonetheless provide significant information about the planet. These include CO2 (which is indicative of a terrestrial atmosphere and has a very strong mid-infrared spectral feature) and CH4 (which has both biotic and abiotic origins). A range of spectral features is needed to characterize Earth-like planet atmospheres.

The James Webb Space Telescope (JWST) (e.g., Gardner et al. (2006)), tentatively scheduled for launch after 2013, will pick up where Spitzer leaves off, in terms of extrasolar planet characterization by primary and secondary eclipse studies. JWST is an infrared telescope with an aperture 6.5 m in diameter, representing a factor of ~60 greater collecting area over Spitzer's 0.85 m diameter aperture. JWST will not only be able to detect thermal emission spectra from hot Jupiters, but also may be able to see emission from hot, super-massive Earths. It may also be possible to perform transmission spectroscopy on such planets with JWST.

NASA's Terrestrial Planet Finder (TPF) missions and ESA's Darwin mission seek to find and characterize Earth-like planets orbiting nearby stars. TPF is split into two separate missions, a visible coronagraph (TPF-C) and an infrared nulling interferometer (TPF-I). Although scheduling and budgets for TPF are tentative, these missions would provide direct imaging of planets and thus low-resolution spectra of a wide variety of planet sizes and semi-major axes. One major goal of these missions would be to search the observed spectra for the biosignature features described above, in the hope of finding evidence for life on another world.

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